Flux Plane Locating in an Underground Drilling System

ABSTRACT

A portable locator and method for establishing the location of the cable line in a region which includes at least one generally straight electrically conductive cable line extending across the region from which cable line a locating signal includes a first arrangement for measuring a local flux intensity of the locating signal at a first above ground point within the region with the portable locator in a particular orientation at the first above ground point. A second arrangement uses the local flux intensity to establish a cable line angular orientation which limits the possible directions to the cable line relative to the particular orientation of the portable locator at the above ground point. A third arrangement uses the measured local flux intensity to establish an actual direction of the cable line that is selected from the possible directions based on certain characteristics of the locating signal.

This is a continuation application of copending prior application Ser.No. 12/646,243 filed Dec. 23, 2009; which is a divisional application ofapplication Ser. No. 11/924,508 filed Oct. 25, 2007 and issued on Feb.16, 2010 as U.S. Pat. No. 7,663,371; which is a divisional applicationof application Ser. No. 11/619,851 filed Jan. 4, 2007 and issued on Dec.4, 2007 as U.S. Pat. No. 7,304,479; which is a continuation ofapplication Ser. No. 11/165,888 filed on Jun. 24, 2005 and issued onFeb. 13, 2007 as U.S. Pat. No. 7,176,690; which is a continuation ofapplication Ser. No. 10/828,391 filed on Apr. 19, 2004 and issued onNov. 8, 2005 as U.S. Pat. No. 6,963,201; which is a continuation ofapplication Ser. No. 10/393,845 filed on Mar. 21, 2003 and issued onJul. 27, 2004 as U.S. Pat. No. 6,768,307; which is a continuation ofapplication Ser. No. 10/285,139 filed on Oct. 30, 2002 and issued onNov. 25, 2003 as U.S. Pat. No. 6,653,837; which is a continuation ofapplication Ser. No. 09/641,006 filed on Aug. 17, 2000 and issued onDec. 17, 2002 as U.S. Pat. No. 6,496,008; the disclosures of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to a system including anarrangement for tracking the position of a boring tool and/or one ormore buried lines within a region and, more particularly, to anarrangement for using certain characteristics of a dipole locatingsignal emanated from the boring tool and/or certain characteristics of alocating signal emanated from the buried lines to provide indications asto the locations of the boring tool and/or the in-ground lines.

The installation of utility lines underground using horizontaldirectional drilling equipment is increasingly popular for reasonsincluding elimination of the need to dig a trench. A number of prior artapproaches are available for the purpose of tracking the position of theboring tool within a region using a dipole locating signal that istransmitted from the boring tool. As one example, see U.S. Pat. No.5,633,589 entitled DEVICE AND METHOD FOR LOCATING AN INGROUND OBJECT ANDHOUSING FORMING PART OF SAID DEVICE (hereinafter the '589 patent) whichis commonly assigned with the present application and which isincorporated herein by reference. The '589 patent, like other prior artapproaches, utilizes a portable locating device to detect the locatingsignal for use in providing positional indications to an operator.

While the '589 patent holds a position representing a significantadvance in the field of boring tool locating and is, in fact, highlyaccurate and effective, it is submitted that there is room foradditional improvement. In particular, at least a minimum degree ofskill is required on behalf of an operator to obtain locating resultsthat are precise within some degree of tolerance. This skill requires,for example, some advance awareness and/or training regarding particularcharacteristics of the locating signal itself. In this connection, atleast some investment in operator training is required. Moreover,locating operations are slowed to some extent by requiring theapplication of procedures which insure accurate locating. Theseprocedures become more important in proportion to the level ofinexperience of an unskilled operator.

As one concern, it should be appreciated that in many regions buriedlines have previously been installed. When such lines are present, it isimportant to avoid damage caused by contact with the boring tool. In thepast, operators often relied on pre-existing information as to thelocation of the lines provided, for example, by a utility company or bylocating services frequently provided by the utility company usingsimplistic locating devices and techniques. This concern has beenfurther elevated with the possibility of the presence of fiber opticcables. In many instances, the lines are themselves electricallyconductive or, in the case of fiber optic cables, are configured with anelectrically conductive tracer wire for use in transmitting a utilitylocating signal therefrom. It is submitted that a portable unit remainsto be seen which facilitates boring tool and cable locating in a single,convenient device.

The present invention provides a highly advantageous portable locatorand associated method which is submitted to resolve the foregoingconcerns while providing still further advantages.

SUMMARY OF THE INVENTION

As will be described in more detail hereinafter, there is disclosedherein a portable locator and associated method for tracking theposition of a boring tool and/or one or more buried lines within aregion.

In one aspect of the invention, the boring tool is moved through theground within a given region along a path while transmitting a locatingsignal such that the locating signal exhibits locate points at thesurface of the ground, both ahead of and to the rear of the boring tool.A local flux intensity of the locating signal is measured for at leastone above ground point to establish a flux vector at that point which isgenerally oriented in a horizontal plane. The orientation of thehorizontal flux vector within the horizontal plane is used in apredetermined way which, at least to an approximation, limits thepossible directions of a nearest one of the locate points relative tothe above ground point.

In one feature, the orientation of the flux vector is used in thepredetermined way by displaying the orientation to an operator at theabove ground point to at least serve as an intermediate step in guidingthe operator to the locate point.

In another feature, the orientation of the horizontal flux vectorindicates two possible directions of the nearest locate point that areopposing with respect to the above ground point. The horizontal fluxvector is used in the predetermined way in combination with a verticalflux intensity of the locating signal that is established at the aboveground point from the measured local flux intensity to indicate a singleone of the two possible directions as the general direction of thenearest locate point.

In still another feature, with the portable locator in a particularorientation, a positional relationship is displayed on the portablelocator including the predicted location of the nearest locate pointrelative to the portable locator having a directional indication suchthat the displayed directional indication points in the actual directionof the predicted locate point from the initial above ground point.

In yet another feature, when the orientation of the portable locator atthe first above ground point is varied from the particular orientation,the portable locator displays an updated positional relationship suchthat the directional indication continuously points to the predictedlocation, irrespective of a predetermined degree of variation oforientation of the portable locator.

In an additional feature, the portable locator is moved iteratively toadditional above ground points at which the display is updated so as toindicate additional directional indications. With sufficient iterations,the location of the predicted locate point converges with the actuallocate point.

In another aspect of the present invention, a local flux intensity ofthe locating signal is measured for at least one above ground point toestablish a flux vector at that point which is generally oriented in ahorizontal plane. An orientation of the flux vector is then displayed toan operator at the above ground point to, at least in part, serve inguiding the operator to at least one of the locate points.

In yet another aspect of the present invention, a boring tool is movedthrough the ground within a given region along a path while transmittinga locating signal having a transmit axis such that the locating signalexhibits locate points at the surface of the ground, one of the locatepoints being ahead of the boring tool and the other one of the locatepoints being to the rear of the boring tool so as define a verticalplane including the transmit axis along with a locate line that isaccessible at the surface of the ground and included in a locate planewhich extends through the boring tool in a direction normal to thetransmit axis. An above ground point is established that is within aside locating region defined between a pair of planes one of whichextends through each locate point normal to the transmit axis. A localflux intensity of the locating signal is measured at the above groundpoint using a portable locator in a particular orientation. Using thelocal flux intensity, a predicted locate line angular orientation isestablished which, at least to an approximation, limits the possibledirections to and orientations of a predicted locate line relative tothe particular orientation of the portable locator at the above groundpoint.

In one feature, the predicted locate line may be in two possiblegeneral, but opposing directions from the above ground point. Thepredicted locate line angular orientation is used in combination with avertical flux plane slope of the locating signal, that is established atthe above ground point from the measured local flux intensity, toindicate a single one of the two possible general directions as thedirection of the predicted locate line.

In another feature, the portable locator is moved iteratively toadditional above ground points at which the display is updated so as toindicate additional positions of the predicted locate line. Withsufficient iterations, the location of the predicted locate lineconverges with the actual location of the locate line. In yet anotherfeature, crossing of the locate line is detected based on monitoring avertical component of flux intensity of the locating signal.

In still another aspect of the present invention, a first type ofpositional relationship is displayed to an operator at an above groundpoint including at least the position of the portable locator and anestimated position of one of a nearest one of the locate points to, atleast in part, serve in guiding the operator to at least the nearestlocate point using measured values of flux intensity of the locatingsignal. After finding at least the nearest locate point, the locator ismoved in a direction from the nearest locate point into an adjacent sidelocating region defined between a pair of planes one of which extendsthrough each locate point and each of which is normal to the transmitaxis. Based on a set of certain criteria within the adjacent sidelocating region, as the portable locator is moved therethrough, theportable locator is switched to a locate line display to display asecond type of positional relationship including at least the positionof the portable locator and an estimated position of the locate line. Inone feature, the locate line display is automatically presented based ondetection of flux lines produced by the locating signal in apredetermined range of flux slope orientation such that the portablelocator is, at least to an approximation, in proximity to the locateline. The predetermined range of flux slope orientation may bedetermined, at least in part, using a vertical component of theintensity of the locating field.

In an additional aspect of the present invention, a region includes atleast one generally straight in-ground cable line extending across theregion, from which cable line a locating signal is transmitted. Thecable line is located by measuring a local flux intensity of thelocating signal at a first above ground point within the region using aportable locator in a particular orientation. Using the local fluxintensity, a cable line angular orientation is established which limitsthe possible directions to the cable line relative to the particularorientation of the portable locator at the above ground point.

In one feature, a vertical flux slope orientation is established at theabove ground point and an actual direction of the cable line is selectedfrom the possible directions based on the vertical flux orientation. Thecable line is then displayed in a position relative to the portablelocator with the portable locator in its particular orientation.

In another feature, an updated positional relationship is displayed onthe portable locator after moving the portable locator into a newparticular orientation to continuously indicate, at least to within anapproximation, the actual position of the cable line, irrespective of apredetermined degree of variation in orientation of the portablelocator.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be understood by reference to the followingdetailed description taken in conjunction with the drawings brieflydescribed below.

FIG. 1 is a diagrammatic perspective see-through view of a portablelocator that is manufactured in accordance with the present inventionand shown here to illustrate its internal components.

FIG. 2 is a diagrammatic plan view of a region including an in-groundboring tool, shown here to illustrate the highly advantageouscharacteristics and use of horizontal flux lines which pass throughfront and rear locate points.

FIG. 3 is a diagrammatic elevational view of the region of FIG. 2showing the boring tool along with flux lines that are oriented in avertical plane along a transmit axis of the boring tool.

FIG. 4 is diagrammatic plan view illustrating the region of FIG. 1including contours of equal total magnetic flux intensity in ahorizontal plane, shown here to illustrate the close relative spacing ofthe contour lines along a locate line.

FIG. 5 is a diagrammatic view of the display of the portable locator ofFIG. 1, showing a horizontal flux vector based on the flux lines seen inFIG. 2 for use in tracking the boring tool using the highly advantageousflux plane technique of the present invention.

FIG. 5A is a diagrammatic view of the display of the portable locator ofFIG. 1, showing the location of a predicted locate point ahead of thecurrent position of the portable locator illustrating the highlyadvantageous and intuitive use of the flux plane locating technique ofthe present invention.

FIG. 5B is a diagrammatic view of the display of the portable locator ofFIG. 1, showing the location of the predicted locate point of FIG. 5Abehind the current position of the portable locator further illustratingthe highly advantageous and intuitive use of the flux plane locatingtechnique of the present invention.

FIG. 5C is a diagrammatic view of the display of the portable locator ofFIG. 1, showing the location of the predicted locate point of FIG. 5A atthe current position of the portable locator such that the predicted andactual positions of the locate point have merged upon having found thelocate point through the operator having intuitively moved the locatepoint to the crosshair intersection.

FIG. 6 is a diagrammatic elevational view of the boring tool in thelocating region after having found the forward locate point shown herefor use in illustrating the determination of distance and depth valuesfrom the locate point.

FIG. 7 is a diagrammatic plan view of the boring tool in the locatingregion shown here to illustrate side locating using a locate line whichis shifted away from a position directly above the boring tool at thesurface of the ground when the boring tool is pitched.

FIG. 8 is a diagrammatic view, in elevation, showing the boring tool ofFIG. 7 and further illustrating, in perspective, a pitch plane that isparallel to the transmit axis of the boring tool and which contains thelocate line, illustrating further details with regard to side locatingusing the locate line.

FIG. 9A is the first figure in a series which diagrammaticallyillustrate the appearance of the display of the locator of the presentinvention during side locating with the predicted locate line ahead ofthe locator at the intersection of the crosshairs.

FIG. 9B is the second in the series of figures beginning with FIG. 9Awhich illustrates the predicted locate line to the left of the locator.

FIG. 9C is the third in the series of figures beginning with FIG. 9Awhich illustrates the predicted locate line to the right of the locator.

FIG. 9D is the fourth in the series of figures beginning with FIG. 9Awhich illustrates the predicted locate line coextensive with ahorizontal one of the crosshairs such that a point along the locate linehas been found while the predicted position of the locate line hasmerged with the actual position of the locate line.

FIG. 10 is a diagrammatic plan view of a locating region including acable line, shown here to illustrate details of a locating signal thatis emitted by the cable line including the way in which the cable linelocating signal resembles the characteristics of a locate line exhibitedwithin a dipole locating field.

FIG. 11 is a diagrammatic elevational end view of the cable line of FIG.10 illustrating further details of the cable line locating signal.

FIG. 12 is a diagrammatic view of the display of the locator of thepresent invention shown here to illustrate the highly advantageoussimultaneous display of a cable line, a locate line and a locate point.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to the drawings, wherein like items are indicated by likereference numbers throughout the various figures, attention isimmediately directed to FIG. 1, which illustrates one embodiment of aportable locator, generally indicated by the reference number 10 andassembled in accordance with the present invention. Locator 10 includesa three-axis antenna cluster 12 measuring three orthogonally arrangedcomponents of magnetic flux in locator fixed coordinates. One usefulantenna cluster contemplated for use herein is disclosed by U.S. Pat.No. 6,005,532 entitled ORTHOGONAL ANTENNA ARRANGEMENT AND METHOD whichis co-assigned with the present application and is incorporated hereinby reference. A tilt sensor arrangement 14 is provided for measuringgravitational angles from which the components of flux in a levelcoordinate system may be determined. Locator 10 further includes agraphics display 16, a telemetry arrangement 18 having an antenna 19 anda microprocessor 20 interconnected appropriately with the variouscomponents. Other components (not shown) may be added as desired suchas, for example, a magnetometer to aid in position determinationrelative to the drill direction and ultrasonic transducers for measuringthe height of the locator above the surface of the ground.

Referring to FIG. 2 in conjunction with FIG. 1, a region 100 is shownincluding a boring tool 102 (indicated using a dashed line) whichtransmits a dipole locating field 104 that is received by antennacluster 12. Region 100 is shown in plan view. It is assumed that thesurface of the region is level and the boring tool is horizontallyoriented. These assumptions are made only for purposes of simplifyingthe present discussion and do not impose restrictions or limitations onthe applicability of the present invention in the absence of theassumptions. It is further assumed that boring tool 102 is oriented suchthat an arrowhead 106 points in a forward direction. Accordingly, aforward locate point 108 and a rear locate point 110 are present at ornear the surface of the region, as described, for example, in U.S. Pat.No. 5,337,002 entitled LOCATOR DEVICE FOR CONTINUOUSLY LOCATING A DIPOLEMAGNETIC FIELD TRANSMITTER AND ITS METHOD OF OPERATION which isco-assigned with the present application and is incorporated herein byreference.

Referring to FIGS. 1 and 2, using orthogonal antenna arrangement 12, twocomponents of the locating signal are established in a horizontal plane,for example, along an x axis 112 oriented in a longitudinal directionalong the locator's axis of symmetry 113 for measuring a b_(x) componentof locating field intensity and along a y axis 114 for measuring a b_(y)component of locating field intensity. A third component b_(z) ismeasured along a vertical, z axis 115. A series of flux lines 116indicate the directional orientation of these flux lines which are madeup of only components of the locating signal intensity within thehorizontal x-y plane of the figure. It is noted that all flux lines inthe horizontal plane (FIG. 2) pass through the locate points. Thislatter characteristic of the locating field is an important recognition,in accordance with the present invention, which is relied on by locator10, as will be further described. The x and y flux components needed toproduce FIG. 2 may readily be determined using locator 10, for example,by insuring that the locator is level and oriented in a particulardirection or by measuring all components using antenna cluster 12 in aparticular orientation (i.e., an x_(ant), a y_(ant) and a z_(ant)component that is orthogonal to the x_(ant)-y_(ant) plane) anddetermining the b_(x) and b_(y) components with reference to tilt andmagnetic sensors.

Turning to FIG. 3 in conjunction with FIG. 2, the flux pattern in avertical plane along drilling direction 106 is shown by FIG. 3 alongwith a locating direction 118 indicated by an arrow. At a locate point,the horizontal components of flux are zero such that the entire magneticflux is vertical. The flux lines in a vertical plane change slope at thelocate points as the locate point is passed. The boring tool is foundbetween the locate points. When the transmitter and the ground surfaceare both level (zero pitch, as illustrated), the boring tool is half wayin between the locate points on a level ground surface 119. Thehorizontal distances between locate points and the over-the-transmitterposition, directly above the boring tool, become unequal when thetransmitter is pitched. Another feature of the dipole magnetic field isa locate line 120 (FIG. 2) at which the magnetic flux lines in allplanes containing the transmitter axis are parallel to the transmitteraxis. Note that the locate line is also in a plane perpendicular to theaxis of the transmitter and, therefore, changes its position relative tothe ground surface with changes in transmitter pitch.

Still considering locate line 120, attention is directed to FIG. 4 whichillustrates region 100 including the total magnetic flux intensity(total signal strength) in a horizontal plane at or above the boringtool, again assuming that the boring tool is at zero pitch. A series ofcontours 122 of total signal strength at different fixed levels ofsignal strength are shown. The contours vary in magnitude of signalstrength by an equivalent amount from one contour to the next. It shouldbe noted that the contours are farther apart along a path in thedrilling direction as compared to their separation along locate line120, normal to the boring tool transmitter axis and drilling direction.Accordingly, it is recognized that the rate of change in signal strengthwith movement at the surface of the ground is greatest along the locateline. In this regard, it should be appreciated that the use of thelocate line is considered as being advantageous in light of thisrecognition, for example, in the side locating technique describedbelow.

Referring to FIGS. 1, 2 and 5, the directional orientation of horizontalflux lines 116 relative to locator 10 is determined using b_(x) andb_(y) measured with respect to the axis of symmetry of the portablelocator. FIG. 5 illustrates display 16 showing region 100 with locator10 at the origins of measured b_(x) and b_(y) intensity vectors.Accordingly, the tangent of an orientation angle α is defined as:

tan α=b _(y) /b _(x)  (1)

Therefore, with locator 10 at rest relative to the surface of theground, equation 1 can be used to display the orientation of a localhorizontal flux line 124 on display 16 of the locator. It should beunderstood that the locator's received flux line (FIG. 5) rotates whenthe locator is rotated about its vertical axis. As long as thetransmitter is at rest, however, the flux line does not move withrespect to the ground surface.

Still referring to FIGS. 1, 2 and 5, in and by itself, the use and/ordisplay of local horizontal flux line 124 is considered to be highlyadvantageous. For example, displaying local horizontal flux line 124 ofFIG. 5 serves to limit the possible directions of the nearest locatepoint relative to the position of the locator. In this regard, it shouldbe appreciated that local horizontal flux line 124 of FIG. 5 representsthe tangent to one of horizontal flux lines 116 shown in FIG. 2. For anyposition of the locator within region 100, the local horizontal fluxvector or flux line may be considered to point to a predicted locationof the locate point. Depending upon the position of the locator in theregion, the predicted location represents the actual location of thelocate point only to an approximation. These considerations are readilyobserved by noting the shape of the horizontal flux lines in FIG. 2 andby bearing in mind that the locator determines the tangent at anyposition along one of flux lines 116. The closer the locator is to thelocate point, the more accurately the tangent of the horizontal fluxline points to the locate point. Upon actually finding a locate point,the horizontal flux components b_(x) and b_(y) go to zero (i.e., onlythe b_(z) component is non-zero at the locate point) and directionalindications are no longer defined.

Referring to FIGS. 2 and 5, using only the described feature ofobserving the local horizontal flux vector or flux line, identifying theposition of a locate point is relatively simple. That is, since x axis112 (FIG. 4) represents the axis of symmetry of the locator, theoperator of the locator may follow one of flux lines 116 by simplyrotating the locator until α is zero. That is, x axis 112 is alignedwith local horizontal flux vector 124. By maintaining this alignment, asat least an intermediate step, while continuously moving the locator inthe predicted direction of the locate point, the locator can follow oneor more of the flux lines directly to a locate point. In this regard, ithas been found that there is no requirement to follow a single one ofthe flux lines since the direction of a predicted location of the locatepoint is always indicated irrespective of being on any particular fluxline. This technique is particularly effective when the locator is inthe region between the locate points, as defined by a pair of sidelocating planes 126 that are oriented such that one plane passes througheach locate point and each is normal to the transmit axis of the boringtool.

Having generally described the flux plane locating feature of thepresent invention, attention is now directed to certain additionalfeatures which further enhance its use. It should be appreciated thatobserving local horizontal flux vector 124 provides no information as towhich direction along the flux vector is the correct direction toproceed in order to arrive at a locate point. That is, the flux vector,by itself, is ambiguous as to the actual direction of the nearest locatepoint along its directional orientation. In fact, if one were to followthe local flux vector initially away from the locate point outside ofside locating planes 126, one would never arrive at a locate point. Asdiscussed above, the technique will always bring one to a locate pointif one begins within the region between side locating planes 126.However, the locate point farthest from the initial position may be theone that is found first in the absence of additional provisions. While anumber of techniques may be used to indicate the direction in which toproceed along the local flux vector towards a nearest one of the locatepoints, one highly advantageous technique utilizes the verticalcomponent of intensity of the locating signal. More particularly, thevertical flux slope, as will be described immediately hereinafter.

Referring to FIG. 3, the slope of a flux line in a vertical plane can bedefined as:

$\begin{matrix}{{\tan \; \beta} = {{- {{sign}\left( \frac{b_{x}}{b_{z}} \right)}}{\sqrt{\left( \frac{b_{x}}{b_{z}} \right)^{2} + \left( \frac{b_{y}}{b_{z}} \right)^{2}}.}}} & (2)\end{matrix}$

Here, the vertical plane is assumed to be along the tangent of thehorizontal flux line at any particular location in region 100. The signof angle β depends on locating direction and on locator positionrelative to the locate points (and locate line). As an example, βbecomes positive when the user is ahead of the forward locate point,pointing the locator in the direction of the boring tool transmitter(i.e., facing the boring tool transmitter in locating direction 118). Asdescribed above, the true direction from locator to locate point cannotbe determined based on the measurement of the flux vector at one pointof the magnetic locating field. Since all horizontal flux lines gothrough the locate points, however, one can just follow a flux line toultimately find a locate point, accepting the attendant 180 degreeambiguity. The use of β, however, serves to resolve this concern.Similarly, the true distance to a locate point can also not bedetermined based on available data obtained at a single point, but theangle β can serve as a practical indication of the distance to thelocate point. By combining the described elements including the use ofthe local horizontal flux vector and the use of β, a highly advantageousand heretofore unseen technique is provided.

By using the technique described immediately above, the operator canprogressively move toward the locate point. After moving to a new aboveground point in the direction of a new predicted location of the locatepoint along the direction defined by the horizontal flux vector at thecurrent location of the portable locator, a new local flux intensity ismeasured at the new above ground point. At the new above groundlocation, the locator then indicates a new direction based on the localhorizontal flux vector. Once again, the operator moves in accordancewith the indications. Ultimately, the operator will arrive at the locatepoint after repeating this process a sufficient number of times, as thepredicted location of the locate point converges on its actual position.At each progressive location, the use of the vertical flux slope is usedto provide the indication as to which of the two directions of the localflux vector is toward the locate point. If the operator passes thelocate point, the locator will indicate that movement in the opposingdirection along the local flux vector is necessary.

With each successive measurement, the locator displays a positionalrelationship including the predicted location of the nearest locatepoint relative to the locator, with a directional indication, such thatthe directional indication points in the actual direction of thepredicted location of the locate point from an initial or subsequentabove ground point. Moreover, it should be appreciated that if thelocator is held at one above ground point in varying orientations, thelocator will display an updated positional relationship which accountsfor variation in the orientation so as provide a directional indicationwhich continuously points in the actual direction of the predictedlocation of the locate point, irrespective of a predetermined degree ofvariation in the orientation. That is, within an approximation,depending upon the frequency with which the display is updated. Using astate of the art microprocessor, an update rate of approximately 15positions per second is readily achieved in locate point indication aswell as in other tracking implementations to be described. Thus, thepositional relationship is continuous insofar as the perceptions of theoperator of the portable locator are implicated, accounting for positionand/or orientation changes experienced by the locator. As will befurther discussed, this locating technique has been found to beremarkably effective in the hands of an operator with virtually noexperience due to the intuitive nature of the display provided by thelocator. Specific displays utilizing the foregoing concepts will beprovided immediately hereinafter.

Turning to FIG. 5A, display 16 is illustrated including a set ofcrosshairs 130 at the intersection of which the locator of the presentinvention is positioned, as indicated by a virtual locator box 10′. Thevertically oriented crosshair is oriented along the axis of symmetry ofthe locator. Hence, the upward direction in the figure represents deadahead to the operator of the portable locator. In essence, this displaypresents a “picture” of the operational region to the operator whichshows his location at the center relative to a predicted direction anddistance of a locate point. The predicted location of a nearest locatepoint is indicated by a target symbol 134. The latter may be connectedto the crosshair intersection by a line 136 (representing the local fluxline) for purposes of emphasizing the predicted direction of the locatepoint. The display of FIG. 5A is provided when) β>0. In this instance,the locate point will be ahead of the locator, above the horizontalcrosshair, with its predicted direction indicated by the localhorizontal flux vector (i.e., the tangent to the local horizontal fluxline given by equation 1). The distance to the predicted location of thelocate point is proportional to |β|. That is, β is zero at the locatepoint and increases with increasing distance from the locate point.

Referring to FIG. 5B, display 16 is illustrated with the predictedlocation of the locate point behind the portable locator. In thisinstance, β<0 such that locate point target symbol 134 is shown in thelower part of the display below the horizontal crosshair. Similarly,distance is based upon the magnitude of β.

Referring to FIG. 5C, display 16 is illustrated in the instance in whichthe locate point has been reached. Thus, β is equal to zero and targetsymbol 134 is moved to the center of crosshairs 130. As mentioned, inapplying this locating procedure, the operator need not follow the sameflux line to the locate point. Instead, the operator may switch over toany convenient flux line which allows moving the locate point targetsymbol to the center of the display crosshairs. Finding a locate pointis as simple as putting a “ball in the box”. In actual testing, thedisclosed technique has proven to be remarkably simple and effective.Moreover, the technique has proven to be so intuitive that locator 10has been handed to a person with no knowledge or training in locatingtechniques and that person has used locator 10 to proceed forthwithdirectly to a locate point.

Referring to FIG. 6, boring tool 102 is shown pitched in region 100.Having reached a locate point, a projected transmitter depth D and ahorizontal distance s at the surface of the ground from the locate pointto an overhead point, OH, directly above the boring tool transmitter areobtained from measured flux and transmitter pitch using the followingequations:

D=r sin(α′+φ)  (3)

s=r cos(α′+φ)  (4)

where r is a radial distance between the boring tool transmitter and thelocate point, φ is the boring tool transmitter pitch, and α″ is theangle formed between the axis of symmetry of the boring tool transmitterand radius r.where

$\begin{matrix}{r^{3} = \frac{4}{{{- {b\sin}}\; \varphi} + {b\sqrt{8 + {\sin^{2}\varphi}}}}} & (5) \\{{\tan \; \alpha^{\prime}} = {\frac{4}{{3\tan \; \varphi} + \sqrt{8 + {9\tan^{2}\varphi}}}.}} & (6)\end{matrix}$

It is noted that b denotes the total locating signal strength at alocate point. When using these equations, if transmitter pitch is notzero, it is necessary to distinguish between the forward and rear locatepoint. For use at the forward locate point, the equations are used aswritten. For the rear locate point, however, the sign of transmitterpitch φ must be reversed. Pitch is generally measured by a sensor in theboring tool transmitter with the pitch data being modulated on thedipole locating signal. It is also worth mentioning that usefuldeterminations can be performed when the horizontal distance from alocate point to the over-the-transmitter position is known, for example,in determining or double checking depth, D. A precise overhead locationcan be determined in one way by monitoring total signal strength, whichreaches a maximum above the boring tool transmitter.

Referring again to FIG. 2, it should be appreciated that there aresituations in which locate points cannot be reached. In thesesituations, the boring tool must be found in an alternative way. Inother situations, the operator my find a locate point, but is stillunable to walk to a position above the boring tool transmitter. Perhapsthe boring tool is beneath a river or some sort of structure. In any ofthese situations, locate line 120 may provide valuable guidance since itis possible that the operator will have access to at least some portionof the locate line.

Referring to FIGS. 7 and 8, characteristics of the locating signal withregard to the locate line will now be described. FIG. 7 is a plan viewshowing boring tool 102 in region 100. FIG. 8 is an elevational viewshowing boring tool 102 at a pitch φ. The surface of the ground is notshown for purposes of clarity. A flux line 150 is illustrated whichpasses through a point A (FIG. 8) on locate line 120. It should beobserved that locate line 120 is shifted in position at the surface ofthe ground due to the nonzero pitch value such that the locate line doesnot pass directly over the boring tool. As the pitch changes, locateline 120 moves at the surface of the ground. A flux vector 152, obtainedfrom a reading taken at point B (offset on the locate line from point A)and on a different flux line (not shown), includes an intensity anddirectional orientation, as indicated. The locate line is itselfcontained in a pitch plane 156 (FIG. 8) that is parallel to axis 157 ofthe boring tool transmitter. Thus, the pitch plane is normal to anyplane which is itself normal to the transmit axis. It should beappreciated that, if the pitch is zero, pitch plane 156 is horizontaland parallel to a horizontal ground surface. Flux vector 152 is resolvedin two orthogonal directions in the pitch plane, which directions aredesignated by the components b_(x) _(p) and b_(y) _(p) . In the instanceof a zero pitch value, the b_(x) _(p) component is equal to thecomponent b_(x) determined along x axis 112 in FIG. 1 while the b_(y)_(p) component is equal to the component b_(y) determined along y axis114. Where the pitch plane is not horizontal, the b_(x) _(p) componentis in vertical alignment with the b_(x) component. In other words, b_(x)_(p) projects directly onto b_(x) within the horizontal x-y plane withthe x axis being the axis of symmetry of locator 10. The b_(x) _(p) andb_(y) _(p) values are readily determined using pitch φ, b_(x) and b_(y).

A predicted locate line orientation angle γ is formed between pitchplane flux vector 152 and b_(y) _(p) . Knowing b_(x) _(p) and b_(y) _(p):

tan γ=b _(y) _(p) /b _(x) _(p)   (7)

Using the predicted locate line angular orientation, γ, locate line 12is displayable as a line normal to the flux line (at point B) in pitchplane 156 (FIG. 8), as will be seen. A measure for the distance to thelocate line may be based upon flux component b_(z) _(p) , normal to thepitch plane. This distance is thus proportional to

$\begin{matrix}\frac{b_{z_{p}}}{\sqrt{b_{x_{p}}^{2} + b_{y_{p}}^{2} + b_{z_{p}}^{2}}} & (8)\end{matrix}$

While the estimated distance and directional orientation are now known,it should be appreciated that two possible, but opposing directionalranges are defined. In other words, it is unknown whether the locateline is in one general direction from the locator or in exactly theopposite direction. Vertical flux slope orientation, β, determined usingequation 2 is useful in combination with the predicted locate lineangular orientation in selecting which of the two opposing directions istoward the actual predicted locate line.

Further considering the use of vertical flux slope orientation in thepresent context, because flux b_(z) _(p) changes sign at the locateline, this information is useful in triggering the side locating orlocate line display. It should be noticed that the described featuresare sufficient to approach the locate line at any angle and to follow itto the transmitter or a reference point when side locating so long asthe locator is between side locating planes 126 of FIG. 2. In thisregard, it should be appreciated that the determined position of thelocate line is a predicted location. As the locate line is approached,the actual position of the locate line converges with the predictionlocation since the relationship defined by equation 7 gives the correctorientation for values measured only at the locate line. Therefore, thistechnique is similar to that discussed above for finding a locate point.That is, the direction of the locate line with respect to the portablelocator is correct within an approximation, depending upon the distancebetween the locate line and the locator such that viewing theorientation of the predicted locate line establishes a general directiontoward the predicted locate line. The side locating technique of thepresent invention is also performed in an iterative manner in order toapproach the actual locate line. After beginning at some initial pointwithin the side locating region, the locator is moved toward thepredicted locate line. The display will then be updated at a new aboveground point to indicate a new predicted locate line in a neworientation based on new readings of the local flux intensity. Theoperator then proceeds in the direction of the new predicted locateline. With sufficient iterations, the positions of the actual locateline and the predicted locate line will converge. Specific displaysutilizing the foregoing locate line technique will be describedimmediately hereinafter.

Referring to FIGS. 9A-9D, a series of locate line displays is presentedwhich diagrammatically illustrate the appearance of display 16 onlocator 10 (FIG. 1). FIG. 9A illustrates the appearance of display 16including a predicted locate line 160 at a distance from the portablelocator that is estimated, for example, using equation 8. Again, theposition of the portable locator in the display corresponds to theintersection of crosshairs 130. In the example of FIG. 9A, the predictedlocate line is directly ahead of the portable locator vertically upwardand skewed in the figure. The dead ahead direction, along the axis ofsymmetry of the portable locator, is emphasized by an arrowhead 162.

FIG. 9B illustrates the appearance of display 16 in the instance wherepredicted locate line 160 is generally to the left of the portablelocator. The operator, in proceeding with the intent of finding thelocate line, will intuitively orient the portable locator such that thepredicted locate line is dead ahead. It should be appreciated that thisside locating technique is highly advantageous in the manner of the fluxplane technique described for locating locate points, as describedabove. That is, a display is presented which illustrates to the operatorthe actual direction in which to proceed toward the predicted locationof the locate line. Moreover, the predicted location of the locate lineis updated at a predetermined interval such that changes in theorientation of the portable locator (i.e., those changes that include arotational component about a vertical axis) result in a new, updatedpresentation on display 16 so as to illustrate the actual direction inwhich the operator should proceed on a continuously updated basisirrespective of orientation and/or position changes to which the locatoris subjected. Again, the display is continuous insofar as the perceptionof the operator is concerned.

FIG. 9C illustrates display 16 with predicted locate line 160 generallyto the right of the position of the portable locator. Accordingly, theoperator will turn to the right, towards the predicted location, whichwill result in a display resembling that of FIG. 9A.

FIG. 9D illustrates display 16 having predicted locate line 160coinciding with the horizontal one of crosshairs 130. Having achievedthis display, the position of the predicted locate line has converged onthe actual location of locate line 120 (FIG. 2). Therefore, the operatorhas succeeded in finding a point on the locate line. Using thisinformation, the operator may continue side locating by continuing totrack the locate line, for example, as the boring tool passes beneath astructure or rock formation.

Because the aforedescribed technique for finding and displaying thelocate line is useful only in the region between planes 126 which definethe side locating region, a number of different methods may be utilizedin order to ensure that the operator is within this region. The methodscan be employed individually or in combination. Three of these methodsare described immediately below.

In a first method, a locate point is first found, for example, using theflux plane technique described above. Thereafter, the operator proceedsin any direction while monitoring the signal strength of the locatingsignal while verifying that the signal strength is increasing. If thesignal strength decreases, the operator should return to the locatepoint and try a new direction. By monitoring for an increase in signalstrength, it is assured that the operator is moving into the sidelocating region defined between side locating planes 126 of FIG. 2.

In a second method, the aforedescribed angle β is used to monitorprogress toward the locate line. Upon approaching the locate line, βapproaches either +90 or −90 degrees. When β is within a pre-selectedrange, the locator is in the vicinity of the locate line at least to afirst approximation. That is, the measurement error in displaying thepredicted locate line is within a corresponding range. In fact, oneimplementation contemplates automatically presenting a locate linedisplay when β enters this pre-selected range. Alternatively, theoperator may be notified on display 16 that the locate line display isavailable for use. The operator may then press a button to manuallyactuate the locate line display mode. As another alternative, the locateline display may automatically be triggered upon detection that thelocate line has been crossed. This is particularly useful in the casewhere the tan β expression of FIG. 2 changes signs. In this instance,the display would then show the predicted locate line behind the locator(i.e., below rather than above the horizontal one of crosshairs 130 inFIG. 9A).

In a third method, the positions of both of the locate points areestablished. The side locating region resides between the locate points,as described above. Having established the locate point positions, theoperator may proceed with the side locating technique of the presentinvention at any point within the side locating region. It should beappreciated that many variations are possible in presenting a locateline display in view of the foregoing teachings. Accordingly, all ofthese variations are considered as falling within the scope of thepresent invention so long as the disclosed teachings are applied.

Having described the highly advantageous features of the presentinvention for establishing locate point positions and for performingside locating, it is important to understand that these features may beintegrated within the same portable locator. An overall highlyadvantageous method is thereby implemented which permits displaying afirst type of positional relationship to an operator at an above groundpoint including at least the position of the portable locator and anestimated position of one of the locate points. This display serves to,at least in part, guide the operator to at least the nearest locatepoint using measured values of flux intensity of the locating signal.After finding the nearest locate point, or both locate points, theportable locator is moved in a direction from the nearest locate pointinto the adjacent side locating region defined between the side locatingplanes, one of which extends through each locate point and each of whichis normal to the transmit axis. Thereafter, as the locator is moved inthe side locating region, a set of certain criteria is monitored withinthe side locating region for actuating purposes. When the criteria aresatisfied, for example, in the aforedescribed manners includingdetection of readings within a predetermined range of flux slopeorientation, switching to the locate line display is performed todisplay a second type of positional relationship including at least theposition of the portable locator and an estimated position of the locateline.

While the described locating procedure and display features allowdetection of the position and orientation of an underground transmitter,detection of an underground utility consisting, for example, of a powercable, a telephone line, a pipeline or any other obstacle with animbedded active tracer wire is performed in a highly advantageous way.This type of locating is referred to hereinafter as cable line locating.In locating a boring tool, described above, the transmitter emits adipole-type magnetic field. In contrast, the current in the tracer wireof a cable line produces a two-dimensional magnetic field, as will befurther described.

Referring to FIGS. 10 and 11, a region 170 is shown including a cableline locating field 172 surrounding a cable line 174 beneath ahorizontal ground surface 175. It should be appreciated that cable linelocating field 172 is much less complex than the magnetic dipolelocating field seen in the methods and examples appearing above. Forpurposes of simplicity, cable line 174 is illustrated in a horizontalorientation, however, this is not a requirement.

Still referring to FIGS. 10 and 11, it is assumed that the cable line isrelatively straight over a distance large compared to its depth beneaththe surface of the ground. Accordingly, FIG. 11 shows that cablelocating field 172 is two-dimensional, characterized by a pattern ofcircular flux lines 176 in planes normal to the tracer wire whichpattern repeats along the cable line. The flux lines immediately abovethe cable line are normal to its plan-view in FIG. 10, as indicated by aflux vector 178. Further, a vertical component of flux intensity, b_(z)(FIG. 11), changes sign at the overhead position directly above thecable line. That is, b_(z) is zero at the overhead position, and istherefore illustrated offset from the overhead position, but has adifferent sign on either side of the overhead position. Clearly, thesecharacteristics of a cable line are related to those of a locate line sothat the locate line procedure, described above, is also generallyapplicable in cable line locating. In this regard, the cable line doesnot exhibit a side locating region which advantageously furthersimplifies the overall cable line locating procedure. The presentexample also considers a horizontally oriented cable line which istypically the case. Further, the instant procedure is directlyapplicable without modification to a slanted cable line.

Still referring to FIGS. 10 and 11, cable line locating signal 172 isresolved into horizontal flux components b_(x) and b_(y). The former isonce again the locate direction that is defined by the axis of symmetryof portable locator 10. In the instance in which the particularorientation of the portable locator is not horizontal, these componentvalues are readily determined, for example, using a tilt meter readingtaken within the portable locator.

A cable line orientation angle γ′ is formed between flux vector 178 andb_(y). Knowing b_(x) and b_(y):

tan γ′=b _(y) /b _(x)  (9)

Using the cable line angular orientation, γ′, locate line 12 isdisplayable as a line normal to flux line 178. In this regard, it shouldbe appreciated that the cable line angular orientation differs withrespect to the aforedescribed side locating procedure in that the cableline orientation angle represents the actual directional orientation ofthe cable line, rather than a predicted direction. Like side locating, a180 degree angular ambiguity is also present. A measure for the distanceof the cable line may be based upon flux component b_(z), normal to thepitch plane. This distance is thus proportional to

$\begin{matrix}\frac{b_{z}}{\sqrt{b_{x}^{2} + b_{y}^{2} + b_{z}^{2}}} & (10)\end{matrix}$

With the estimated distance and an ambiguous directional orientation nowknown, the directional ambiguity is resolved using vertical flux slopeorientation, β, determined using equation 2. Further, considering theuse of vertical flux slope orientation in the cable line context,because flux b_(z) changes sign at the locate line, this information isuseful in indicating passing across an overhead point directly above thecable line. Like a locate line, the cable line can be approached at anyangle and followed thereto. The cable locating technique of the presentinvention is also performed in an iterative manner in order to find thecable line. After beginning at some initial point, the locator is movedtoward the indicated cable line. The display will then be updated at anew above ground point to indicate a new position of the cable line in anew orientation based on new readings of the local flux intensity. Theoperator then proceeds in the direction of the new cable line location.With sufficient iterations, the cable line will be found.

Specific displays utilizing the cable line locating technique of thepresent invention are not provided since these displays may beessentially identical to the side locating displays seen in FIGS. 9A-9D,described above. As in the case of side locating using this displayform, the operator, in proceeding with the intent of finding the cableline, will intuitively orient the portable locator such that thepredicted cable line is dead ahead. Moreover, the display presentedillustrates to the operator the actual direction in which to proceedtoward the location of the cable line. Like the side locatingimplementation, the displayed location of the cable line is updated at apredetermined interval such that changes in the orientation of theportable locator, including a rotational component about a verticalaxis, result in a new, updated presentation on display 16 so as toillustrate the actual direction in which the operator should proceed ona continuously updated basis. As mentioned above, this display featureis operative with respect to position and/or orientation changesexperienced by the locator and is, for practical purposes, continuous atthe contemplated update rates.

Turning to FIG. 12, the cable line locating feature and associateddisplay may be used alone for cable line locating or may be integratedwith the locate point and/or locate line locating features. FIG. 12illustrates display 16 with a combination of all three forms of locating“objects” displayed. Specifically, a cable line 180 is represented by asolid line, a predicted locate point 182 is illustrated as a dot and apredicted locate line 184 is shown as a dashed line. All of these areindicated in relation to a “virtual” locator 10′. To avoidmisinterpretation, the cable line symbol on the display of the portablelocator differs from the appearance of the locate line. Textual labelsmay also accompany any of the illustrated items. By simultaneouslyshowing both the cable line and transmitter data, the portable locatorof the present invention provides a powerful tool for use in avoidingthe hazards of drilling into underground utilities while guiding aboring tool.

A number of possible approaches may be used in order for the locator todistinguish between the signal from the boring tool transmitter and oneor more buried cable lines. For example, a different frequency may beused for each item being located or tracked. The same tri-axialreceiving antenna 12 (see FIG. 1) may be used to receive all theemployed frequencies. A digital signal-processing receiver is used toextract the signal amplitudes for each antenna and frequency.Alternatively, different sets of receiving devices may be used. In anycase, a microprocessor is configured for processing the data fordisplay. In the case of AC power cables, the signal naturally emanatingfrom the AC cable may be used as the cable locating signal. If more thanone cable is present, however, a different frequency for each cable maybe employed. The location data for each cable may be presented alone orin combination with the location data for other cables.

One alternative to multiple frequency use is the use of time multiplexedsignals that are synchronized at the locator. Such multiplexing may beused for the cable lines in the ground or may include the boring tooltransmitter as well. Combinations of multiple frequencies and timemultiplexing are also contemplated.

Because the portable locator and associated method disclosed herein maybe provided in a variety of different configurations and modified in anunlimited number of different ways, it should be understood that thepresent invention may be embodied in many other specific forms withoutdeparting from the spirit or scope of the invention. For example, thepresent teachings are applicable to any type of locating which utilizesa dipole locating field or a cable line type locating field. Therefore,the present examples and methods are to be considered as illustrativeand not restrictive, and the invention is not to be limited to thedetails given herein, but may be modified within the scope of theappended claims.

1. In an overall system in which a transmitter is moved through theground within a given region along a path while transmitting a locatingfield such that the locating field exhibits a pair of locate points atthe surface of the ground, one forward of the transmitter and the otherto the rear of the transmitter, a method for guiding an operator to eachof the locate points using a portable locator, said portable locatorincluding a display and said method comprising: a) measuring theintensity of the locating field with the portable locator at an initialabove ground point and with the portable locator in a particularorientation; b) using the measured intensity of the locating field todetermine a predicted location of a nearest one of the locate points inproximity to the portable locator at the initial above ground point; andc) with the portable locator in said particular orientation, displayingon the portable locator a positional relationship including thepredicted location of the nearest locate point relative to the portablelocator having a directional indication to serve in guiding an operatorto the nearest locate point.
 2. The method of claim 1 including pointingthe directional indication in an actual direction of the predictedlocation of the nearest locate point.
 3. The method of claim 1 whereindisplaying the predicted location of the nearest locate point includesestimating a horizontal distance between the predicted location of thenearest locate point and the portable locator and displaying theestimated distance on the portable locator.
 4. The method of claim 3wherein the estimated horizontal distance is determined using a verticalplane flux line slope.
 5. The method of claim 4 wherein the verticalplane flux line slope is determined by the expression${- {{sign}\left( \frac{b_{x}}{b_{z}} \right)}}\sqrt{\left( \frac{b_{x}}{b_{z}} \right)^{2} + \left( \frac{b_{y}}{b_{z}} \right)^{2}}$where b_(x) is one horizontal component of flux intensity at the aboveground point, b_(y) is another horizontal component of flux intensitywhich is orthogonal to b_(x), and b_(z) is an established vertical fluxintensity component.
 6. The method of claim 1 wherein the orientation ofthe portable locator at the first above ground point is varied from saidparticular orientation and wherein said method further comprises: d)displaying on the portable locator an updated positional relationshipsuch that the directional indication continuously points to thepredicted location irrespective of a predetermined degree of variationof orientation of the portable locator.
 7. The method of claim 1 furthercomprising: d) moving the portable locator in the direction of thepredicted location of the nearest one of the locate points; e) at anadditional above ground point, measuring the intensity of the locatingfield with the portable locator in a desired orientation; f) using themeasured intensity of the locating field at the above ground point todetermine an additional predicted location of the nearest one of thelocate points in proximity to the portable locator at the additionalabove ground point; and g) with the portable locator in said desiredorientation, displaying on the portable locator the additional predictedlocation of the nearest locate point relative to the portable locatorincluding an additional directional indication such that the displayedadditional directional indication points in the actual direction of theadditional predicted location from the additional above ground point. 8.The method of claim 7 further comprising: h) repeating steps (d) through(g), repeatedly moving the portable locator in the direction of theadditional predicted location of the nearest one of the locate pointsuntil the additional predicted location of the nearest locate pointconverges with the actual location of the nearest locate point.
 9. Themethod of claim 8 further comprising: i) measuring the pitch of thetransmitter with the portable locator positioned at the nearest locatepoint and measuring a locate point value of the flux intensity; and j)using the measured value of the locating field and the pitch, estimatinga horizontal distance, S, of the transmitter from the nearest locatepoint and a depth, D, of the transmitter beneath the surface of theground.
 10. The method of claim 9 wherein D and S are determined by theexpressionsD=r sin(α′+φ)s=r cos(α′+φ) where φ is pitch, b is a total flux intensity determinedusing the measured locating field at the locate point and where r and α′are determined by $\begin{matrix}{{r^{3} = \frac{4}{{{- {b\sin}}\; \varphi} + {b\sqrt{8 + {\sin^{2}\varphi}}}}},{and}} \\{{\tan \; \alpha^{\prime}} = {\frac{4}{{3\tan \; \varphi} + \sqrt{8 + {9\tan^{2}\varphi}}}.}}\end{matrix}$
 11. In an overall system in which a transmitter is movedthrough the ground within a given region along a path while transmittinga locating field such that the locating field exhibits a pair of locatepoints at the surface of the ground, one forward of the transmitter andthe other to the rear of the transmitter, a portable locator for guidingan operator to each of the locate points, said portable locatorcomprising: a) a receiver for measuring the intensity of the locatingfield; b) a processor for using the measured intensity of the locatingfield to determine a predicted location of a nearest one of the locatepoints in proximity to the portable locator at an above ground point;and c) a display for displaying on the portable locator a positionalrelationship including the predicted location of the nearest locatepoint relative to the portable locator having a directional indicationto serve in guiding an operator to the nearest locate point.
 12. Theportable locator of claim 11 wherein said processor and said display areconfigured to cooperate to point the directional indication in an actualdirection of the predicted location of the locate point from theportable locator.
 13. The portable locator of claim 11 wherein theprocessor is configured for estimating a horizontal distance between thepredicted location of the nearest locate point and the portable locatorfor display by the display arrangement.
 14. The portable locator ofclaim 13 wherein the processor is configured for estimating thehorizontal distance based on a vertical plane flux line slope.
 15. Theportable locator of claim 14 wherein the vertical plane flux line slopeis determined by the expression${- {{sign}\left( \frac{b_{x}}{b_{z}} \right)}}\sqrt{\left( \frac{b_{x}}{b_{z}} \right)^{2} + \left( \frac{b_{y}}{b_{z}} \right)^{2}}$where b_(x) is one horizontal component of flux intensity at the aboveground point, b_(y) is another horizontal component of flux intensitywhich is orthogonal to b_(x), and b_(z) is an established vertical fluxintensity component.
 16. The portable locator of claim 11 wherein theorientation of the portable locator at the first above ground point isvaried from a particular orientation in which the intensity of thelocating field is initially measured and wherein said processor isconfigured for updating the positional relationship such that thedisplayed directional indication of the display arrangement continuouslypoints to the predicted location irrespective of a predetermined degreeof variation of orientation of the portable locator.
 17. The portablelocator of claim 11 wherein the processor is configured for using anestablished pitch of the transmitter along with a measured locate pointvalue of the flux intensity, taken at the nearest locate point, toestimate a horizontal distance, S, of the transmitter from the nearestlocate point and a depth, D, of the transmitter beneath the surface ofthe ground.
 18. The portable locator of claim 17 wherein D and S aredetermined by the expressionsD=r sin(α′+φ)s=r cos(α′+φ) where φ is pitch, b is a total flux intensity determinedusing the measured locate point value at the nearest locate point andwhere r and α′ are determined by $\begin{matrix}{{r^{3} = \frac{4}{{{- {b\sin}}\; \varphi} + {b\sqrt{8 + {\sin^{2}\varphi}}}}},{and}} \\{{\tan \; \alpha^{\prime}} = {\frac{4}{{3\tan \; \varphi} + \sqrt{8 + {9\tan^{2}\varphi}}}.}}\end{matrix}$